There are many applications wherein metallic materials are used for their strength and endurance and are therefore subjected to loads, stresses, strains, and other forces which, over time, may have a tendency to fatigue the material and create a risk of catastrophic failure. It would be highly desirable to be able to test these discrete metallic parts in situ for their state of fatigue such that they might be replaced or renewed prior to any such catastrophic failure. In still other instances, and especially for critical applications involving health and safety, standards have been established for the routine testing of certain metallic parts prior to their being placed in service to ensure against any such failure of the part. In those applications, techniques have been developed and are available in the prior art to achieve such testing. These include such things as x-ray, destructive testing of selected parts from a lot, and other techniques, all of theses techniques being well known in the art. However, these techniques are all subject to certain drawbacks such as expense, inconvenience, and in some cases failure to entirely eliminate the possibility of premature failure of the part. Still another situation in which these kinds of tests for fatigue are conducted involve many instances where materials or parts are welded and the integrity of the weld must be verified prior to the equipment being placed in service. One particular application, from amongst many, involves the federal safety standards which govern the construction of nuclear power plants. Certain welds in certain critical equipment contained within the plant are subjected to x-ray and other kinds of testing in order to verify their integrity prior to the plant being placed in service. A nuclear power plant presents perhaps an extreme example of the potential harm which might befall not only the people involved but the public at large should a critical piece of equipment suffer a premature failure. There are a myriad of other applications perhaps considered not as critical but which also are important to the health and safety of many people, including the public at large.
Despite the fact that testing for fatigue has been utilized for some time, and the relationship of damping to fatigue has been well known for some time, the inventor is not aware of any other efforts in the prior art to utilize the relationship of damping to fatigue in the arena of fatigue testing. For example, in a paper presented at a colloquium on structural damping at the ASME Annual Meeting in Atlantic City, N.J. in December of 1959, the phenomenon of "plastic strain" was analyzed. In particular, damping was used as a parameter for determining the interrelationship between stress history and stress amplitude as mechanisms for affecting plastic strain in a material. As concluded in the paper, at low stresses and intermediate stresses, within 1-50% of a fatigue limit, damping was not seen to be affected by the stress history of the material. On the other hand, at high stresses, typically above 50% fatigue limit, where large plastic strain damping may be observed, stress history played a part in affecting plastic strain, as measured by the damping factor. Stated differently, data were presented indicating that at low and intermediate stress, the damping factor does not change with the number of fatigue cycles. However, above a critical stress, damping increases with the number of fatigue cycles thereby indicating that stress history plays a part in plastic strain under these conditions. Although this article treated the interrelationship between stress history and stress amplitude, and their effect on damping (plastic strain), there was no disclosure or suggestion of utilizing a measured damping factor as an indicator of the state of fatigue of a material. As stated therein, the article focused on how stress history and amplitude might produce a particular damping factor but not how a measured damping factor could be used as a predictor of relative fatigue in a part. See Structural Damping edited by Jerome E. Ruzicka, ASME Proceedings, 1959.
In order to solve these and other problems in the prior art, and as a departure from the teachings in the prior art, the inventor herein has succeeded in developing the technique of measuring the damping factor of a discrete piece of metallic material, such as a part in an assembly or the like, and using that damping factor for determining the fatigue integrity of that part either by comparing it with a standardized damping factor or with previously measured damping factors for the same part. The part might be a single piece of material, or it might be a welded or otherwise joined piece of material and the test may be one for integrity, i.e. cracking, voids, or the like, as might be required for a new part, or the test might be conducted for determining the fatigue in the part after having been installed and used over time. For new part testing, it is anticipated that standardized damping factors may be determined and available for comparison with the measured damping factor for the new part. Alternately, the damping factor of a series of identical new parts might be measured and used to cull out those new parts which evidence signs of early fatigue and failure, or cracks, voids, or other defects in manufacture. After a part has been installed and used over a period of time, a damping factor measurement may be made periodically to determine the part's increasing fatigue. This technique may be used to identify parts which are in need of replacement prior to any chance of catastrophic failure. There are other applications and situations in which the damping factor measurement of a discrete piece of metallic material might be used to good advantage. These particular examples are being given as exemplary.
In making the damping factor measurement, the inventor herein has also succeeded in developing a simple but effective and accurate technique for measuring the damping factor using either of two methods. Utilizing a first method, an impulse of energy may be applied to the part, such as by striking it with a blunt object or the like, and the induced vibration in the part measured by a transducer which converts the vibration into an electrical signal for input to a computer. A computer may then easily make the appropriate calculation from the induced vibration to determine the damping factor. Generally, as is known in the art, the damping factor of a part vibrating at its natural frequency may be determined by comparing peak amplitudes of successive cycles of the vibration. In an alternative method, a continuous input of energy may be provided to the part instead of an impulse of energy. In a preferred embodiment, a frequency generator may be coupled to a transducer, such as a speaker, shaker, or other such device, and the frequency generator tuned or adjusted so as to sweep through the range of the lowest natural frequencies of the part. As the input of energy remains constant, the part would continue to vibrate at its natural frequency such that the damping factor may be readily calculated by measuring the half-power bandwidth of a cycle and dividing it by the center frequency, as is well known in the art. Using either of these methods, a vibration is induced in the part and the response thereto is measured from which the damping factor is determined.
One of the advantages of using the inventor's method of inducing a vibration in the part is that it is believed that the part need not be isolated and may be tested in place. This eliminates disassembly of the part from any larger assemblage which dramatically reduces any costs involved in using the present method in determining the damping factor. This provides great advantages over other prior art methods which require disassembly and isolation of the part to be tested, such as in the x-ray method. Furthermore, the device used to implement the method disclosed herein may be relatively compact, readily portable, and sufficiently small such that the testing of many differently sized parts which might be otherwise relatively difficult to access may be readily tested.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.